This invention relates generally to Positron Emission Tomography (PET) systems and, more particularly, to calibration of PET systems.
A PET system typically includes a PET scanner and a control unit to control the scanner. The PET scanner includes detector blocks used to detect annihilation photons. With time and usage, the PET scanner requires a number of calibration operations to ensure optimal and proper performance. The different types of calibration operations include, for example, detector gain calibration, detector mapping calibration, signal level calibration, timing delay calibration and coincidence sensitivity calibration.
Some of the known calibration operations, such as, detector gain calibration, detector mapping calibration, signal level calibration, and timing delay calibration, take about an hour, while others, such as coincidence sensitivity calibration, take 10-12 hours. The calibration operations may be performed either with a rotating pin or with a fixed source of positrons. In the case of a rotating pin, a pin with a positron-containing source is rotated along the edges of a detector in the field-of-view to generate data that is used in calibrating the PET scanner, referred to as calibration data.
Known calibration operations are carried out manually by an operator and based on a recommended schedule. The operator also may initiate the calibration operations if there is a change in the state of the scanner and the scanner is operating below its optimal performance. To assist the operator in this determination, a quality assurance procedure is often established in which some data is acquired by the scanner and analyzed to determine if the calibration state is sufficient for patient imaging.
However, this manual method is only effective in determining the state of the system when the quality assurance data is acquired, which is typically once per day before the first patient is imaged. If the state of the scanner changes during the course of the day, that change may go unnoticed unless the operator initiates the quality assurance procedure at some other time. Further, continuous advances in technology lead to an increasingly demanding nature of calibration requirement for future applications. For example, more accurate calibration is required for smaller windows for timing or energy, for example in Time of Flight (TOF) scanners. Such a level of calibration is not guaranteed by the design of the scanner.